Preheat the oven to 160°C. Pat the lamb dry with paper towels. In a bowl, stir together the yoghurt, fruit chutney, cumin, coriander, curry powder, turmeric, salt and pepper to make a thick paste. Rub the paste all over the lamb, coating it generously.
Place the halved garlic bulb, quartered onions, and ginger slices in the base of a roasting dish, and sit the lamb on top. Pour ½ cup of hot water into the dish, cover tightly with foil, and roast for 3½-4 hours, or until the meat is very tender and pulls apart easily with a fork.
While the lamb cooks, make the raita by mixing the yoghurt, mint, coriander, lemon juice and cumin together. Season with salt and pepper and set aside.
For the final 30 minutes of cooking, remove the foil to allow the top of the lamb to caramelise. Rest the lamb for 20 minutes before carving or pulling apart. Scatter with the herbs and pomegranate arils (if using), and serve warm with the raita.
